1. Field of the Invention
The present invention relates to switching power source apparatuses used for consumer appliances and audio equipment, and particularly, to a switching power source apparatus capable of stably operating even on input and output variations and preventing magnetostrictive noise.
2. Description of the Related Art
FIG. 1 is a circuit diagram illustrating a switching power source apparatus according to a related art. This apparatus performs a pseudo resonant operation. In FIG. 1, a rectifier 1c and a smoothing capacitor 1d form a rectifying-smoothing circuit 1 that rectifies and smoothes an AC voltage from an AC power source into a DC voltage. Both ends of the smoothing capacitor 1d are connected to a series circuit that includes a primary winding P of a transformer 2 and a switching element 3 made of a MOSFET. A current detecting resistor 9 (current detector) detects a current passing through the primary winding of the transformer 2 or the switching element 3 and outputs a current detected signal to a turn-off controller 25a. 
Both ends of a secondary winding S of the transformer 2 are connected to a series circuit including a diode 4 and a smoothing capacitor 5. The smoothing capacitor 5A provides a DC output voltage Vout. The diode 4 and smoothing capacitor 5 form an output rectifying-smoothing circuit. A voltage detector 7 detects a voltage across the smoothing capacitor 5, i.e., the DC output voltage Vout, finds an error between the detected DC output voltage Vout and a reference voltage, and sends the error as an error signal to the turn-off controller 25a on the primary side.
A controller 8 generates a drive signal that controls an ON/OFF period of the switching element 3 so as to substantially keep the DC output voltage Vout constant. The controller 8 includes a power source start/stop circuit (Reg+Start/Stop) 24, the turn-off controller 25a, a bottom detector 41, and an R-S flip-flop 23.
The power source start/stop circuit 24 activates each part with a voltage from the smoothing capacitor 1d passed through a resistor 10, and after the activation, operates each part with a voltage from an auxiliary winding D rectified and smoothed through a diode 11 and a capacitor 12. The power source start/stop circuit 24 also has a function of stopping each part.
The turn-off controller 25a generates an OFF signal to turn off the switching element 3 according to the error signal from the voltage detector 7 and the current detected signal from the current detecting resistor 9 and sends the OFF signal to a reset terminal R of the R-S flip-flop 23. The bottom detector 41 serves to reduce a switching loss when the switching element 3 turns on. According to a voltage generated by the auxiliary winding D of the transformer 2, the bottom detector 41 detects a bottom in an oscillation of a drain-source voltage Vds of the switching element 3, generates an ON signal to turn on the switching element 3, and sends the ON signal to a set terminal S of the R-S flip-flop 23.
The switching power source apparatus of FIG. 1 that performs a pseudo resonant operation increases, in principle, a switching frequency under light load, to deteriorate efficiency.
The global warming in recent years requires energy saving measures such as efficiency improvement to be taken. FIG. 2 is a circuit diagram illustrating a switching power source apparatus disclosed in International Patent Application Publication No. WO2004/023634. This apparatus carries out a bottom skip operation. Namely, as illustrated in FIG. 3, the turn-on timing of a switching element 3 is delayed under light load with the use of the ringing of a drain-source voltage Vds of the switching element 3 during an OFF period of the switching element 3, to thereby extend the OFF period, suppress an increase in a switching frequency, decrease a switching loss, and improve efficiency under light load.
The switching power source apparatus of FIG. 2 consists of an externally excited flyback DC-DC converter having a controller 8. Operation of this apparatus will be explained.
Under heavy to normal load, an output signal VLD of a D flip-flop 28 is high as illustrated in FIG. 3(E). In synchronization with a first fall edge of an output signal VBD (FIG. 3(D)) of a bottom detector 41, an output terminal Q of a first D flip-flop 50 of a bottom skip controller 42 outputs a single pulse of signal VDF1. As a result, in synchronization with the first fall edge of the output signal VBD of the bottom detector 41, an AND gate 52 outputs a single pulse of AND signal VAD that increases to a high level.
At this time, an output terminal Q of a second D flip-flop 51 of the bottom skip controller 42 outputs a low-level signal VDF2. Accordingly, in synchronization with the first fall edge of the output signal VBD of the bottom detector 41, an OR gate 53 outputs a single pulse of OR signal VOR that increases to high to set an R-S flip-flop 23.
As illustrated in FIGS. 3(D) and 3(C), in synchronization with the first fall edge of the output signal VBD of the bottom detector 41, a drive signal VG provided by the R-S flip-flop 23 to a gate terminal of the switching element 3 changes from low to high to turn on the switching element 3.
At this time, a drain current ID (FIG. 3(B)) to the switching element 3 linearly increases and a voltage VOCP at a connection point between level shifting resistors 17 and 18 linearly decreases below a high reference voltage VDTH as illustrated in FIG. 3(F). When the voltage VOCP reaches the voltage level of a detection signal VFB from an output voltage detector 7, a current mode controlling comparator 20 outputs a high-level signal V2 to reset the R-S flip-flop 23.
As results, the drive signal VG from the R-S flip-flop 23 to the gate terminal of the switching element 3 changes from high to low as illustrated in FIG. 3(C), to change the switching element 3 from ON to OFF. In this way, under heavy to normal load, a pseudo resonant operation is carried out to turn on the switching element 3 at the time when the transformer 2 completely discharges flyback energy and the drain-source voltage Vds of the switching element 3 reaches a minimum point (bottom point).
If the load becomes lighter, the output signal VLD of the D flip-flop 28 changes from high to low as illustrated in FIG. 3(E). Then, as illustrated in FIG. 3(B), a maximum value of the drain current ID to the switching element 3 slightly becomes higher, and as illustrated in FIG. 3(F), a peak of the voltage VOCP at the connection point of the level shifting resistors 17 and 18 slightly moves downward.
At this time, a voltage level changer 31 changes a reference voltage supplied to a non-inverting input terminal of a current detecting comparator 27 from the high reference voltage VDTH to a low reference voltage VDTL as illustrated in FIG. 3(F). At this time, in synchronization with a second fall edge of the output signal VBD of the bottom detector 41 illustrated in FIG. 3(D), the output terminal Q of the second D flip-flop 51 of the bottom skip controller 42 outputs a single pulse of signal VDF2.
Since the AND gate 52 outputs a low-level signal VAD, the OR gate 53 outputs, in synchronization with the second fall edge of the output signal VBD of the bottom detector 41, a single pulse of OR signal VOR that increases to high to set the R-S flip-flop 23.
Consequently, the drive signal VG supplied to the gate terminal of the switching element 3 from the R-S flip-flop 23 in synchronization with the second fall edge of the output signal VBD of the bottom detector 41 changes from low to high as illustrated in FIGS. 3(D) and 3(C), to turn on the switching element 3. The drain current ID to the switching element 3 linearly increases as illustrated in FIG. 3(B) and the voltage VOCP at the connection point of the level shifting resistors 17 and 18 linearly decreases.
At this time, the detection signal VFB from the output voltage detector 7 is higher than the low reference voltage VDTL as illustrated in FIG. 3(F), and therefore, the voltage VOCP at the connection point of the level shifting resistors 17 and 18 does not reach the low reference voltage VDTL. When the voltage VOCP reaches the level of the detection signal VFB, the current mode controlling comparator 20 outputs a high-level signal V2 to reset the R-S flip-flop 23.
As a result, the drive signal VG from the R-S flip-flop 23 to the gate terminal of the switching element 3 changes from high to low as illustrated in FIG. 3(C), to change the switching element 3 from ON to OFF. In this way, the bottom skip operation is carried out under light load, to turn on the switching element 3 at a second minimum point of the drain-source voltage Vds of the switching element 3 during an OFF period of the switching element 3.
FIG. 4 illustrates an oscillation state with respect to a load ratio of the externally excited flyback DC-DC converter having the controller 8 of FIG. 2. The “load ratio” is a ratio of power consumed by load to power provided by the converter to the load. A load ratio of 50% to 100% corresponds to normal to heavy load under which the pseudo resonant operation is carried out. A load ratio of 0% to 70% corresponds to normal to light load under which the bottom skip operation is carried out.
When the load ratio decreases from 100% to 50%, the pseudo resonant operation is shifted to the bottom skip operation, which is continued up to no-load state such as a standby state in which the load ratio is 0%. If the load ratio changes from 0% to 70%, the bottom skip operation is shifted to the pseudo resonant operation, which is continued up to a load ratio of 100%.
Under light load, the switching power source apparatus of the related art illustrated in FIG. 2 uses the bottom skip controller 42 to turn on the switching element 3 at every second minimum point of the drain-source voltage Vds of the switching element 3. This elongates an OFF period of the switching element 3 and decreases the switching frequency of the switching element 3. Namely, the number of times of switching of the switching element 3 decreases to reduce a switching loss under light load and improve the conversion efficiency of the switching power source apparatus for a wide range of load.
Under light load, flyback energy of the transformer 2 is supplied within a relatively short period after the turning-off of the switching element 3 to a load (not illustrated) from the secondary winding 2b through the output rectifying-smoothing circuit 6. This produces narrow-width voltage pulses (Vds) containing free oscillation portions between the drain and source of the switching element 3 as illustrated in FIGS. 3(A) and 4(A).
Accordingly, when the load is light, the bottom skip controller 42 carries out the bottom skip operation to turn on the switching element 3 whenever the bottom detector 41 detects a second minimum point in the drain-source voltage Vds. The bottom skip operation elongates an OFF period of the switching element 3 and decreases the oscillation frequency thereof.
Under heavy to normal load, flyback energy of the transformer 2 is supplied within a relatively long period after the turning-off of the switching element 3 to the load (not illustrated) from the secondary winding 2b through the rectifying-smoothing circuit 6. This generates wide-width voltage pulses (Vds) between the drain and source of the switching element 3.
Accordingly, when the load is heavy to normal, the bottom skip controller 42 carries out the pseudo resonant operation to turn on the switching element 3 whenever the bottom detector 41 detects a first minimum point in the drain-source voltage Vds. The pseudo resonant operation changes the switching element 3 from OFF to ON when flyback energy of the transformer 2 is completely discharged and the drain-source voltage Vds of the switching element 3 reaches a minimum point (bottom point).